Doppler processing has been applied extensively in shipboard, land based and airborne radars for the improvement of signal-to-clutter ratios and the estimation of target radial velocity. (See U.S. Pat. Nos. 4,484,194 and 4,489,392). For radars with fully coherent exciter and transmitter stages, digital Doppler processing is typically achieved through quadrature phase detection, A/D conversion and vector processing. However, the implementation of Doppler processing in radars utilizing noncoherent transmitter (such as a magnetron) is more complex since the pulsed oscillator transmitter produces a random phase on each pulse. This aspect has limited the application of Doppler techniques in several important classes of radars, including shipboard surface surveillance radars, coastal surveillance radars and ship collision avoidance radar systems. By using the latest advances in high speed conversion and digital timing circuits, a simpler method of extracting Doppler data from noncoherent systems is proposed in accordance with the present invention, making Doppler processing for these systems more cost effective.
Doppler processing of radar data is accomplished by taking the data for several consecutive transmissions and filtering the data to enhance the range samples that exhibit a phase change caused by the Doppler effect of a moving target. According to this prior technique, the amplitude and phase of a given range cell must be precisely measured for several radar transmissions, which implies that the exact amplitude and phase of the transmitted pulse of energy is known. For radars which use a pulsed high power oscillator as a transmitter, this is not the case. The amplitude is well controlled but the phase of the transmitted signal is random from transmission to transmission.
In the past, Doppler processing for this type of radar involved the use of a device called a COHerent Oscillator (COHO), which is a stable oscillator at the radar IF, that is phase locked to the transmitted signal every transmission. A COHO circuit is illustrated in the Prior Art circuit of FIG. 1. The COHO output is used to translate the IF return signal to In-phase (I) and Quadrature (Q) base band signals where they are amplified and converted to digital words for filtering by the Moving Target Indicator (MTI) filer. Examples of typical MTI filters are found in U.S. Pat. Nos. 4,153,899; 4,293,856; and 4,394,658. The performance of the system is limited by the stability and balance of the COHO and associated quadrature video processing and conversion circuits (not shown, but see U.S. Pat. No. 4,488,154). To obtain adequate data for separating true moving targets from clutter these circuits are complex and costly.
The performance of any Doppler processing system is limited by the accuracy to which the amplitude and phase of the data is measured. This determines to what extent the clutter signals can be separated from targets containing Doppler and this can be expressed as decibels of clutter attenuation.